Inflammation is an important aspect of the natural defense process. Inflammation becomes a pathological process, requiring medical intervention, when inflammatory mediators cause excessive damage to the surrounding tissue. Examples of such pathological processes are rheumatoid arthritis (RA) and psoriasis. Recently, a significant inflammatory component has been found in other types of disease, for example, neurological disorders such as multiple sclerosis and Alzheimer's disease. A common feature of many inflammatory diseases is an elevation in phospholipase A2 (PLA2) activity.
PLA2 is the common name for a diverse group of enzymes that specifically hydrolyze the sn-2 bond of glycerophospholipid to release free fatty acids and lysophospholipids. PLA2 is thought to be rate limiting in the release of arachidonic acid. The other product of its reaction, lysophospholipid, is thought to be the precursor of platelet activating factor (PAF). PAF and the arachidonic acid metabolites, eicosanoids, are pro-inflammatory lipid intermediates derived from mobilized cell membrane phospholipids by the action of phospholipase enzymes. PLA2 is thus implicated as having a crucial role in the production of the entire cascade of phospholipid-derived inflammatory mediators.
For persistent inflammation three classes of drugs are widely used, corticosteroids, non-steroidal anti-inflammatory drugs (NSAIDs) and slow acting disease modifying drugs.
Corticosteroids are the most potent and effective agents in controlling inflammatory conditions. Unfortunately, prolonged use of these drugs is associated with side effects. Topical corticosteroid preparations are widely used for inflammatory dermatological conditions and inhaled corticosteroids account for 55% of the asthma market in the United Kingdom. Prednisolone is the most commonly administered corticosteroid in RA, which though possibly affecting the underlying disease process does not provide a cure and is associated with severe side-effects.
NSAIDs relieve the symptoms of inflammation without altering the course of the disease, but they have adverse gastrointestinal and renal side effects. Their main action is inhibition of arachidonate cyclooxygenase (in inflammatory cells the COX2 isoenzyme) and thus inhibiting prostaglandin and thromboxane production.
Disease-modifying anti-rheumatic drugs (DMARDs) such as gold products, auranofin (SKB), chloroquine (Sanofi), sulfasalazine (Pharmacia & Upjohn), cyclosporin (Sandoz (Novartis)) and methotrexate (MTX, APH, Pharmacia & Upjohn) are second-line agents for treatment of RA. In most cases their mode of action is ill defined and the term ‘slow-acting’ is applied because these agents may take weeks or months to have demonstrable effect. Treatment with DMARDs has to be continued for years. If complete remission is achieved for at least six months, the dosage is gradually reduced and may be stopped altogether. DMARDs appear to decrease radiographic joint damage and improve acute-phase markers in RA but they all have adverse effects.
Diclofenac ([o-[(2,6-dichlorophenyl)amino]phenyl]acetate) is a non-steroidal anti-inflammatory drug of the phenylacetic acid class. When given orally the absorption of diclofenac is rapid and complete. It binds extensively to plasma albumin. Substantial concentrations of drug are attained in synovial fluid, which is the proposed site of action of the NSAIDs. Diclofenac is a potent inhibitor of prostaglandin synthesis and has also been shown to inhibit interleukin-1 (IL-1β) and tumor necrosis factor alpha (TNF-α), involved in osteoarthritis. Gastrointestinal complications such as ulceration and intolerance are the most common adverse effect of diclofenac. Renal dysfunction and hypersensitivity reactions also occur. Many patients with rheumatic disorders have some degree of renal function impairment and are especially susceptible to the induction of renal failure by NSAIDs.
Other non-steroidal anti-inflammatory drugs such as salicylates, indomethacin and ibuprofen directly inhibit cyclooxygenase, a key enzyme in the synthesis pathway of prostaglandins. However, since these drugs inhibit early reactions in the arachidonic acid metabolism, they may block the formation of more than one product, hence leading to severe side effects. Indomethacin, for example, may also disrupt calcium flux across membranes, inhibit cAMP-dependent protein kinase and phosphodiesterase.
It would, therefore, be desirable to be able to extend the usefulness of NSAIDs to conditions that do not respond to lower doses of the drugs and to reduce undesirable side effects by their targeting to the diseased cells.
The use of prodrugs to impart desired characteristics such as increased bioavailability or increased site-specificity for known drugs is a recognized concept in the state of the art of pharmaceutical development. The use of various lipids in the preparation of particular types of prodrugs is also known in the background art. However, none of the background art discloses prodrugs comprising NSAIDs that upon activation by intracellular lipases enable preferential accumulation and release of the drug within the diseased cells.
International Patent Application WO 91/16920 discloses phospholipid prodrugs of salicylates and nonsteroidal anti-inflammatory drugs wherein the drug is directly linked, without any spacer, to either or both of the glycerol hydroxyls of a phospholipid or to available hydroxyls or amines of phospholipid head groups. These prodrugs, when taken orally, protect the gastric mucosa and release the active principle in the gut via the action of pancreatic enzymes.
In other examples of phospholipid prodrugs, the formulation of the prodrugs into liposomes or other micellar structures is the feature that enables their preferential uptake, for instance by liver cells or by macrophages as in the case of the phospholipid conjugates of antiviral drugs disclosed in International Patent Applications WO 93/00910 and WO 90/00555.
International Patent Application WO 96/22780 discloses compositions comprising nonsteroidal anti-inflammatory drugs non-covalently associated with zwitterionic phospholipids. In contrast, the present invention relates to nonsteroidal anti-inflammatory drugs covalently bound to a phospholipid via a spacer group.
U.S. Pat. No. 5,149,794 discloses a method for delivering drugs selectively to intracellular organelles. The disclosed compounds comprise an antiviral or antineoplastic drug covalently bound to a lipid carrier via a spacer group which may act to modulate drug release at the target site. In contrast to the present invention, the disclosed prodrug is site specific due to the existence of the lipid carrier, and drug release from the lipid conjugate is not a requirement for the drug targeting. In addition, said U.S patent does not disclose phospholipids as the lipid carriers, nor compounds comprising nonsteroidal anti-inflammatory drugs.
U.S. Pat. No. 5,256,641 discloses methods of delivering and specifically targeting antigenically-active peptides to cells for the specific production of immunological reactivity against such peptides. In contrast, the present invention does not disclose prodrugs wherein the active ingredients are peptides, though peptides may serve as a spacer between the active drug and the phospholipid.
U.S. Pat. No. 5,543,389 discloses covalent polar lipid-drug conjugates for facilitating the entry of drugs into cells at pharmokinetically useful levels. The rationale for specific activation of the prodrug in that case, are very different from the present invention. The examples of the present invention with phospholipids were not made and with hindsight it is clear that it would be ineffective to synthesize active phospholipid prodrugs if the spacer between the lipid and the drug is less than a specific length of at least 4 carbon atoms, because of unfavorable conditions due to steric hindrance and stereochemical problems.